It is frequently necessary in oil field operations to connect or disconnect lengths of drill pipe which are threaded together at their joints. Power tongs replace the old system of wrapping a chain five or six times around drilling pipe and then pulling on the chain in order to make-up or to break a threaded joint in a string of oil well drilling pipe. Power tongs contain multiple assemblies nested within each other to engage with the pipe and force the pipe to turn. A typical power tong includes a mechanism for gripping the external surface of a first pipe section and then rotating this first pipe section while the adjacent pipe section, to which it is connected, is held stationary or rotated in the opposite direction.
The outer frame of an open-throated power tong is held stationary, typically by being chained to the oilrig. This frame has a C-shaped access opening to receive pipe. In use, a gate is swung across the opening to close the gap and make the frame rigid. Some examples of power tongs with various types of gates to close the outer frame are U.S. Pat. No. 6,279,426, U.S. Pat. No. 6,058,811, U.S. Pat. No. 5,144,868, and U.S. Pat. No. 4,593,584. Inside the outer frame are sets of pivoting jaws contained within an internal, rotating ring gear, also C-shaped. This ring gear is forced to rotate through its geared outer periphery. The interior surface of the ring gear carries a camming surface, which, as the ring gear rotates with respect to the jaws, causes the jaws to swing inwardly to engage with a drill pipe, casing, tubing, or equivalent positioned therein.
Typically, such a power tong assembly includes top and bottom plates, cage plates, which are rotatably carried by the ring gear. These cage plates in turn carry hinge pins to support the pipe engaging jaws. The top and bottom cage plates are constrained, so that they are marginally free to rotate with respect to the ring gear when the jaws are not fully engaged with a drill pipe. Thus, when the ring gear is forced to rotate through its geared outer periphery while the cage plates and hinge pins thereon are constrained, the camming surface of the ring gear may advance the jaws for engagement with the drill pipe. Once the jaws have fully engaged the drill pipe, the constraints on the cage plates are removed, allowing the cage plates to move in unison with the ring gear and pipe engaged therein.
When the jaws have forcibly engaged a pipe, an outwardly-directed radial reaction force is applied by the jaws to the camming surface of the ring gear. This reaction force must be resisted by the face of the camming surface of the ring gear in order for a rotational torque to be applied to the drill pipe. The reaction force on the camming gear surface increases as the rotational torque requirement increases. The more rotational torque required, the greater the jaw pressure against the pipe and the greater the outwardly-directed force against the ring gear's camming surface. This reaction force will tend to spread the ring gear open. Under previously used moderate torque levels, the rotating gear structure has been made sufficiently stiff that this tendency to spread has been tolerable However, at extremely high torques, this tendency for spreading of the rotating gear increases. High torques are becoming increasingly common in the industry.
To resist such spreading, open-throated power tongs typically include containment rollers mounted either on each of the cage plates or within the frame of the tong to bear against and resist spreading of the ring gear. These containment rollers transfer the spreading force to the tong body, and in some cases the body is not sufficiently rigid to effectively restrict spreading to tolerable limits. It has therefore been recognized that additional means to resist spreading of the ring gear are needed.
In existing power tongs a door is pivotally connected to the tong frame or body adjacent a side of the open throat. This door is positioned to extend across the open throat when in the closed position, resisting the tendency for spreading to occur across the entire tong body.
U.S. Pat. No. 5,904,075, asserts that it describes a method of reducing transmission of spreading forces to the ring gear. In this reference, portions of the jaws are advanced into a hooked connection to each other so that a portion of the spreading forces developed by the hinge pins of the jaws are absorbed through the jaw contacts rather than being focused exclusively on the pivot pins. By this arrangement the jaw pivot pins are partially restrained against spreading by the hooked engagements extending between the jaws. This reduces the spreading forces that would otherwise be directed into the cage plate and ring gear.
The invention described in this patent helps to lower the outwardly-directed force acting on the ring gear by transmitting a portion of the load through the cage plates. But no support is provided to the cage plates to prevent such cage plates from spreading.
There remains a need for a power tong system that includes cage plates that are better able to resist spreading themselves as well as provide support for the ring gear to resist expansion when exposed to the large amounts of torque developed in modern power tongs.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.